When the versatility issue or multiple different tones are concerned, I prefer to take the simpler, non-patch, non-midi, non-programmed, non-whatchamacallit way. What I think a versatile amp should be able to do/have is:
1) A responsive distortion channel. Should deliver hi-gain sounds but still clean up nicely, not thinning out, when rolling down the vol knob on your guitar. Vol boost for rock solos would be nice as well.
2) Headroom. A wide dynamic range, controllable with your pick. I really dig having sort of a saxophone-like dynamic range, going from whisper to a shout in one phrase. Still working on that a bit...
3) A good clean channel. Thick, round, rich clean sound with some crunch on top if necessary. Wide, not thin.
With all that, you get pretty far regardless of your style.
Well, if you really need to sound like you have 15 amps onstage at once and don't have the money to spend on the real thing, then Line6 might be ok... although their stuff hasn't really convinced me in live situations. Their usefulness in project studios is a fact. But if you need lots of tones, get a good amp and use your head & hands instead of programming...
My Rivera delivers all the goods mentioned in the list above. The Mesa I had was tighter beyond all recognition, but didn't have much variety when tweaking the guitar's vol knob. I've heard lotsa good things about Mark IV, though.